Effects of Buffer Acceptors on Breakdown Voltages of AlGaN/GaN HEMTs with a High-k Passivation Layer

AlGaN/GaN HEMTs are now receiving great interest for application to high-power microwave devices and high-power switching devices. However, the breakdown voltage is known to be greatly lower than that theoretically predicted. To improve the breakdown voltage, the introduction of field plate is shown to be effective both experimentally and theoretically. But the introduction of field plate may increase the parasitic capacitance, leading to degrading the high-frequency performance. In a previous work, as another method to improve the breakdown voltage, we proposed a structure including a high-k passivation layer, and showed that the breakdown voltage increased significantly. We assumed an undoped semi-insulating buffer layer where a deep donor compensates a shallow donor. Recently, Fe- and Cr-doped semi-insulating buffer layers are often adopted and they acts as deep acceptors. Therefore, in this work, we analyze AlGaN/GaN HEMTs with a buffer layer where a deep acceptor above the midgap is considered, and studied whether the breakdown voltage is improved by introducing a high-k passivation layer. As a result, we have found that the breakdown voltage improves as in a case with a deep donor whose energy level is set equal to the acceptor’s energy level, and also the breakdown voltage becomes higher in the region where the permittivity of the passivation layer is high. In addition, it has been shown that the breakdown voltage becomes higher in the high-k region when the deep-acceptor’s energy level is deeper, because the buffer leakage current becomes smaller.

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